Color-selection mask and post-deflection focus assembly for a color tube



J n- 7. 1 69 J. A. CHRISTENSEN 3,421,048

COLOR-SELECTION MASK AND POST-DEFLBCTION FOCUS ASSEMBLY FOR A COLOR TUBE Filed Aug. 18, 1967 [NVE/VIOR. John AChrlsten sen Attorney United States Patent 9 Claims ABSTRACT OF THE DISCLOSURE A three color picture tube has a screen formed of a repeating series of phosphor strips having in each series strips that emit red, blue and green light. The parallax mask for color selection is a grid of vertically disposed wires and an electrically conductive mesh is placed on opposite sides of and spaced from the wire gn'd. These three elements serve as lens electrodes with the mesh electrodes maintained at the potential of the screen but the grid electrode established at a lower potential chosen to provide, in conjunction with the mesh electrodes, a system of converging electron lenses which focuses an electron beam passing through the assembly onto selected portions of the screen.

Background of invention The color tube currently in comercial use is of the shadow mask variety in which an apertured mask positioned between the gun mount and the screen achieves color selection by permitting each of the three beams issuing from the gun mount to see only those incremental portions of the screen which have the phosphor colors assigned to be excited by each such beam. The transmission through the aperture mask is low and imposes an undesired limit to the brightness capability of the tube.

As explained in copending application Ser. No. 571,533 filed on Aug. 10, 1966, in the name of Sam H. Kaplan and assigned to the assignee of the present invention, post acceleration techniques have been proposed to effect focusing of the electron beam at the mask apertures to increase brightness but they give rise to unwanted secondary emission and other problems which mitigate against their use. A different approach which avoids the difiiculties of previous post acceleration tubes is described in the Kaplan application which features a mask assembly having the properties of a unipotential lens. Structurally, in one embodiment the aperture mask serves as the middle of the three electrodes of such a lens. Aligned with the aperture mask and spaced on opposite sides thereof are two other electrodes which have the same aperture pattern as the mask and are electrically isolated from it. If the outer electrodes are operated at the same direct current potential as the screen while the inner one is at a different potential, preferably a lower value, the three electrodes from a unipotential lens at each aperture of the mask. Adjustment of the lens parameters, especially the operating voltages, permits the beams to be focused on the screen. This greatly enhances the transmission of the electron beam through the mask assembly, leading to a marked improvement in screen brightness. The present invention is a similar solution to the brightness problem of the parallax mask color picture tube and is predicated on an unexpected improvement that results from employing a conductive mesh as at least one of the lens electrodes.

Summary of the invention It is a principal object of the invention to provide a new and improved color-selection mask and post-deflection force assembly for a color cathode-ray tube.

It is another and particular object of the invention to simplify the construction of the parallax mask and postdefiection focus assembly of a color cathode-ray tube.

A color-selection mask and post-deflection focus assembly constructed in accordance with the invention is utilized to improve the brightness of a color cathode-ray tube having a gun mount and a color phosphor screen. The assembly comprises three lens electrodes arranged in substantially parallel relation with respect to one another and with respect to the screen. At least one of these electrodes serves as a color selection parallax mask having a pattern of apertures for directing electrons from the gun mount to particular portions of the screen dependent on the angle at which the electrons enter the assembly. At least another of the lens electrodes is formed of a metallic mesh having interstitial dimensions that are small compared with the dimensions of the apertured electrode. There are means for establishing the end ones of the lens electrodes at a common direct-current potential substantially equal to that of the screen to create a field-free space at least between the assembly and the screen but preferably also from the assembly to the final electrodes of the gun mount. This same means establishes the intermediate lens electrode at a direct-current potential difference relative to the end electrodes to constitute therewith a system of convergence electron lens to focus electrons exiting from the assembly to the aforementioned particular portions of the screen.

In one embodiment, the parallax element is a grid of conductive wires for providing color selection for a line or strip type phosphor screen. Metallic meshes are arranged on opposite sides of the wire grid and are operated at screen potential.

Brief description of the drawing The features of the present invention which are believed to be novel are set forth with particularity in the appended claims. The invention, together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawings, in the several figures of which like reference numerals identify like elements, and in which:

FIGURE 1 represents a line or strip type three-color cathode-ray tube having a color-selection mask and postdeflection focus assembly constructed in accordance with the invention;

FIGURE 2 is a structural detail of that assembly;

FIGURE 3 is a fragmentary representation of a modification of the mask and focus assembly;

FIGURE 4 represents a predistorted lens electrode that may be used in practicing the invention; and

FIGURE 5 shows the effect of an electrostatic force in overcoming the predistortion of the element of FIG- URE 4.

Description of the preferred embodiments The cathode-ray tube of FIGURE 1 is for the production of images in simulated natural color and has a glass envelope 21 terminated at one end in a faceplate 22 which serves as an image screen. The reference character 23 indicates the deposit of phosphor elements on the inner surface of faceplate 22. For a three-color system, the screen has interlaced or interleaved deposits of red, green and blue emitting phosphors with the individual deposits of a configuration suited for the type parallax mask or barrier to be employed. Where the parallax mask is a plate having a pattern of round holes, which is the most common form in current commercial use, each phosphor deposit has the configuration of a dot and the interlacing provides what are known as dot triads. On the other hand, where the parallax element is in the form of a gird of parallel wires or conductors, as is true of the particular embodiment under consideration, the screen is of the strip type, that is to say, the red, blue and green phosphors are deposited in vertical strips that repeat across the image field and define line-type triads. Over the phosphors is applied the usual layer 24 of conductive and light reflecting material, such .as aluminum. Of course, this layer is pervious to electrons as required to attain excitation of the phosphor deposits.

At the opposite end, envelope 21 terminates in a neck portion which houses a gun mount 25 including an array of three electron guns for the tri-color tube under consideration. The guns are similar to one another and are of well known construction, individually serving to develop and direct an electron beam along a predetermined path and through a beam deflection region toward screen 22. Since the gun structure is well known and constitutes no part of the present invention, it has not been shown in detail. It will include the usual beam convergence assembly and will have contact elements 26 for contacting the aquadag coating 27 customarily deposited on the interior surface of the conical section of the envelope. The ultor voltage is applied from a power source to an anode button (not shown) sealed into envelope 21 and connecting to the final electrodes of each gun in the mount through aquadag 27 which also extends to the conductive coating 24 of the screen. This establishes all such components at a common direct-current potential.

The deflection region is a portion inclosed by a deflection yoke 28 having line and field scanning coils suitably energized to establish deflection fields for causing the electron beams to scan screen 22 in a series of fields of parallel lines in a manner well understood in the art. Adjacent deflection yoke 28 is a convergence yoke assembly 28a which is energized by suitably shaped convergence signals having components at line and field fre quencies to accomplish dynamic convergence of the three electron beams as they are caused to be deflected over the image field of screen 22.

Interposed between screen 22 on the one hand and the beam deflection region adjacent yoke 28 on the other is a color selection mask and post-deflection focus assembly 30 to be described in detail hereafter but which, for the moment, may be considered simply as the wire grid type of parallax barrier or mask. It has a pattern of apertures related to the pattern of phosphor deposits on screen 22 for admitting electrons to particular portions of the screen dependent on the angle at which those electrons enter the assembly. More particularly, the parallax element in the usual way may be said to permit each of the three guns of gun mount 25 to excite but a single assigned color phosphor of each triad on screen 22 as the beams are scanned across the parallax element.

The power supply which is obviously required to apply potentials to the electrode system of the tube and other signal sources for modulating the beams and energizing yokes 28, 28a have also been omitted from the drawing since they of themselves constitute no part of the invention and are well known both as to construction and performance.

As thus far described, the arrangement of FIGURE 1 is a conventional three-gun, three-color stripped screen picture tube utilizing a grid of parallel wires for color selection. Its operation is well understood and need be reviewed only summarily.

Each of the three beams from gun mount 25 is modulated by both luminance and chrominance information and is directed through the deflection field of the tube toward screen 22, reaching the screen by passing through the apertures or interconductor spacings of parallax element 30. The deflection fields established by yoke 28 cause these beams to scan the screen in a series of fields of parallel lines and during the scansion, the convergence field established by yoke 28a maintains the beams converged even though they are deflected to travel from side to side and top to bottom of screen 22. In this way, the electron beams synthesize an image in simulated natural color on screen 22 in well-known fashion.

Consideration will now be given to the structure of assembly 30 arranged, in accordance with the invention, to improve the transmission in respect of electrons of the scanning beams in order to achieve increased brightness. It will be understood that a screen pack may be employed in which the stripped phosphor screen and a color-selection and focusing assembly of the type designated 30 are made as one component and supported within the tube envelope. Where this is done, the faceplate of the tube is clear and the elements of the screen pack are planar. It is preferable, however, to adopt the arrangement of FIG- URE 1 wherein the screen is deposited on the faceplate and assembly 30 is secured internally of the envelope in a desired space relation to the screen.

In this case, the faceplate is approximately cylindrical although it does have a curvature in the horizontal plane and a smaller curvature in the vertical plane, forming in fact a toroidal section and, the structural elements of assembly 30 are essentially cylindrical sections. More particularly, the assembly comprises three lens electrodes in parallel spaced relation with one another and with screen 22, one electrode serving as the parallax mask and at least another being formed of a conductive metallic mesh. Specifically, the lens electrodes are aligned along but disposed traversely of the longitudinal axis of the tube and the center electrode 31 is a grid of parallel conductive wires which may be made of stainless steel disposed in the same direction as the phosphor strips of screen 22. For the case under consideration, they extend in a vertical plane. The interconductor spacings of the grid provide apertures for emitting electrons from gun mount 25 to particular portions of the screen 22 dependent on the angle at which the electrons enter assembly 30. The remaining two electrodes 32 and 33 are formed of a metallic mesh having interstitial dimensions small compared with the aperture dimensions of the wire grid. Broadly speaking, these electrodes may be any sort of electron permeable metal foil but practical embodiments use a metallic mesh of most any type, woven, knitted electro-formed or expanded metal such as nickel. Generally, the focus voltage required is independent of the coarseness or type of mesh but the quality of the focus is a function of mesh density, coarser meshes giving a poorer condition of focus. Increasing the mesh density tends to sharpen the focus and degrees of focus are obtainable beyond that required for optimum operation of the tube. Experience indicates a mesh of lines per inch to be an acceptable compromise for a 21" rectangular tube.

In order to maintain fixed and uniform spaced relations for electrodes 31-33, they are mechanically mounted under tension; certainly this is well understood in the art with respect to the wire grid type of parallax element. Tensioning is accomplished by employing conductive frames that border the periphery of the electrodes and the conductors 31 as Well as the meshes 32, 33 are mechanically secured to their respective frames While under tension. The tension here under consideration is simply that required to have the electrodes assume and retain the configuration imposed by the shape of their respective frames and is not to be confused with the condition discussed hereafter in which the mesh of at least electrode 32 is under such tension as to distort the lens element from the configuration it is to exhibit in the operation of assembly 30.

The fragmentary view of FIGURE 2 is taken in a vertical plane showing a portion of frame 31a of grid 31 and also showing frames 32a, 33a of meshes 32, 33 respectively. It is highly desirable that the peripheral portions of the central electrode 31 be shielded and this is accomplished by a conductive housing 34 secured to frames 32a, 33a and maintained at the same direct-current potential. Interposed between frame 31a and housing 34 are a series of spaced insulators 35 which integrate these components into one subassembly. One or more of the insulators, shown in FIGURE 2, may be similar to the wellknown feed through type of capacitor having a conductor projecting axially of the insulator and extending through an aperture in housing 34 to be insulated therefrom and yet permit the application of an excitation potential to the conductors of grid 31. It is desirable that the insulator 35 be made long to increase the structural path length between mesh electrodes 32, 33 and grid 31 for the purpose of minimizing the possibility of voltage breakdown. The tendency to such breakdown or cold emission of electrons is a function of focal length which determines the electric field between the lens electrodes and is essentially independent of the separation of the electrodes measured in a direction of beam travel. Mounting studs provided within envelope 21 in cooperation with mountmg fixtures (not shown) of the frame structure may be employed to secure assembly 30 in position in any convement manner.

Geometrical considerations of the three guns in mount 25 and wire grid 31 accomplish color selection by well known parallax principles and the desired focusing functioning is accomplished by means for establishing appropriate direction current potentials on the lens electrodes. More particularly, the end electrodes 32, 33 are established at a common direct current potential with screen 22 and the conical section of envelope 21. To this end, a contact 40 connects shield housing 34 and mesh electrodes 32, 33 to the aquadag coating 27 of the tube. This causes the mesh electrodes to be at the same potential as the conductive backing layer 24, aquadag layer 27 and, through snubbers 26, the final electrodes of the three guns in mount 25. Because of this energizing arrangement, assembly 30 is located in a field-free space.

The intermediate or central electrode 31 is established at a direct-current potential difference relative to mesh electrodes 32, 33 to constitute therewith a system of convergence electron lenses to focus electrons exiting from assembly 30 onto preselected portions of screen 22. Terminal 41 which is in electrical connection with grid 31 extends through insulator 35, housing 34 and envelope 21 to facilitate the application of potential to wire grid 31. Of course, where it passes through the tube envelope, it must not be in contact with the aquadag coating 27. Wire grid 31 is maintained at a lower direct-current potential than electrodes 32, 33 which is required in order to have a convergence lens system with the illustrated arrangement. This is also advantageous both from the standpoint of circuitry and because it allows operation at a lower value of focus differential.

If the appropriate focusing potential difference of grid 31 relative to electrodes 32, 33 is established, a converging electrode lens is established by each pair of conductors of grid 31. There is one strip or line triad, that is to say, a strip of red, blue and green emitting phosphor, of screen 22 associated with each such lens and the lens parameters are selected so that each beam is focused on the strip of the triad to which it is assigned. This condition obtains for all positions of the beams within the scanning raster and greatly improves the transmission of assembly 30 for electrons of the beam as required to achieve improved brightness.

Upon the application of focusing potentials to the lens electrodes an electrostatic attractive focus tends to cause bowing of meshes 32 and 33 which is to be avoided since for optimum operation the lens elements should have a uniform separation or spacing from one another. The adverse effect of bowing may be minimized by increasing the weight of the mesh or by increasing the separation of the mesh electrodes from grid 31 but on the other hand Center to center spacing of conductors of grid 31 inch 0.40 Conductor diameter of grid 31 do 0.003 Interelectrode spacing at the axis of the tube inch .125

Mesh of electrodes 32, 33 l.p.i

Radius of lens electrodes inches 38.2

Horizontal radium of screen 22 do 34.4

Vertical radius of screen 22 do 78 Distance from center of assembly 30 to screen 22 on the tube axis (approximately) inch 1 A tube utilizing such an assembly would typically have a screen or ultor voltages of 25 kv. and the voltage of grid 31 would then be about 21.8 kv. Such a tube would have a brightness increase of approximately three times compared with the shadow mask color tube in current commercial use. The transmission of assembly 30 would be 55 percent compared with the 17 percent transmission of the shadow mask tube.

The focal length D of each convergence lens within assembly 30, measured along the path of the beam as determined experimentally is closely in accordance with the following:

lcV d 0 0 E (:05 cos y in which k is a constant, V is the potential of meshes 32 and 33 which is also equal to the anode in ultor voltage, V is the potential of grid 31, and d is the center to center spacing from grid 31 to either electrode 32 or 33. 6 and B are the angular coordinates in the horizontal and vertical planes respectively expressing the angle of the beam in relation to the local normal to the surface of grid 31. This formula is for the arrangement of FIGURE 1 in which grid 31 is mid-way between meshes 32, 33 and reduces to the well-known Davisson-Calbick equation when 0 =0 =0.

Equation 1 shows the manner in which focal length varies with deflection angle which contributes a particularly attractive feature of this arrangement with respect to degrouping. Since the faceplate 22 has curvature in both the horizontal and vertical planes, the spacing of assembly 30 with respect to the screen is less at the borders of the screening raster than in the center. This change in spacing is in the direction required to compensate for degrouping errors of the screen. At the same time, as Equation 1 indicates, the focal power increases and the focal distance decreases at the borders of the scanning raster, corresponding to the change in spacing of assembly 30 relative to screen 22. Since the variation in grid-to-screen spacing is in the correct sense for both degrouping correction and proper focusing, there is no need for compromising these two properties. Moreover, the described arrangement provides an additional degree of freedom in satisfying the necessary requirements for degrouping correction and constancy of beam focus over the raster. It is apparent from Equation 1 that for a given voltage difference and for a fixed anode voltage, the focal length varies in linear relation with the grid to mesh spacing d. Consequently, this spacing may be adjusted by contouring of the mounting frames to optimize the operation of the tube with respect to focus.

While it is preferred that the wire grid serving as the parallax element be positioned between the mesh electrodes as illustrated in FIGURE 1, this is not a necessary restriction. As indicated schematically in FIGURE 3, assembly 30 may be arranged to have a pair of wire grid electrodes 31', 31 with an interposed Wire mesh electrode 32', here shown as also having reinforcing rods 32". Of course, in utilizing the embodiment of FIGURE 3, it is essential that the conductors of wire grids 31, 31" be accurately aligned along the path of beam travel.

Since electrostatic forces are necessarily present when operating potentials are applied to the lens electrodes, there is inevitably some tendency to bowing of the mesh elements which may be minimized by increasing the space separation of the electrodes or by the addition of reinforcing elements to increase their mechanical strength as stated above. Additionally, the mesh electrode may be prestressed or predistorted in a sense which is complementary to the effects of bowing. For example, if an electrode to be subject to bowing is predistorted, the electrostatic force giving rise to bowing may be relied on to cause that element to assume a desired physical configuration in the operation of the lens. More particularly, there is illustrated in FIGURE 4 a metallic mesh electrode of the cylindrical type, that is to say, it is curved as a simple circular arc in a horizontal plane but in the orthogonal or vertical plane it is linear or straight. The configuration, of course, is imposed by the shape of the horizontal and vertical frame numbers 45 and 46 respectively and if the mesh 47 is attached to the frame under a sufficient horizontal tension as indicated by force arrows T the electrode does not exhibit a cylindrical configuration but instead experiences a predistortion as shown in FIGURE 4. However, when the electrode is positioned in the lens and subjected to the electrostatic force represented by arrows F of FIGURE 5, the predistortion may be overcome and the element may then assume the desired cylindrical surface indicated in FIGURE 5.

When utilizing a predistorted mesh electrode in the selection-mask and focus-deflection assembly of FIGURE 1, the predistorted element is utilized as the lens electrode on the gun side of the structure. This reduces the adverse effect of bowing that may be experienced where the lens element facing the gun is a regular cylindrical section in the absence of the bowing effect attributable to the fields of the lens. Assuming that the lens electrode facing the screen is also a metallic mesh as described in conjunction with the arrangement of FIGURE 1, further relief from any adverse effect of bowing may be realized by changing the position of the wire grid or barrier electrode so that it is closer to the mesh electrode facing the gun than to the mesh electrode facing the screen.

While it is not a simple matter to illustrate, predistortion, may also be utilized Where the mesh electrode is carried by a toroidal frame, that is to say, one in which the frame elements are simple circular arcs in both the horizontal and vertical planes and the radius of curvature is greater in the vertical plane. The metallic mesh is prestressed by the application of both horizontal and vertical focus as it is attached to the toroidal frame and is thereby predistorted. This distortion may be compensated or overcome, resulting in the desired toroidal electrode form, under the influence of the electrostatic field established at the time operating potentials are applied to the lens.

There area number of advantages resulting from the described col wr-selection mask and post-deflection focus assembly. There is no beam warpage so that simple exposure techniques may be utilized in screening. Manufacturing tolerances to construct the assembly, especially the embodiment of FIGURE 1, are most reasonable since there is nothing critical about the relative positioning of lens electrodes 31-33. The voltage difference required for focusing represent-s almost a four fold decrease over alternative arrangements. There is freedom for adjusting the focus across the raster and, most importantly, a brightness gain of nearly three times is realized over the shadow mask tube.

While particular embodiments of the invention have been shown and described, it will be obvious to those skilled in the art that changes and modifications may be made without departing from the invention in its broader aspects, and, therefore, the aim in the appended claims is to cover all such changes and modifications as fall within the true spirit and scope of the invention.

I claim:

1. A color-selection mask and post-deflection focus assembly for a color cathode-ray tube having a gun mount and a color phosphor screen comprising:

three lens electrodes in substantially parallel spaced relation with each other and with said screen, at least one of said electrodes constituting a color selection parallax mask having a pattern of apertures for admitting electrons from said gun mount to particular portions of said screen dependent on the angle at which said electrons enter said assembly and at least another of said electrodes being formed of an electrically conductive metallic mesh having interstitial dimensions small compared with the aperture dimensions of said apertured electrode;

and means for establishing the end ones of said electrodes at a common direct-current potential substantially equal to that of said screen to establish a fieldfree space between said assembly and said screen, and for establishing the intermediate one of said electrodes at a direct-current potential difference relative to said end electrodes to constitute therewith a system of converging electron lenses to focus electrons exiting from said assembly onto said particular portions of said screen.

2. A color-selection and post-deflection focus assembly in accordance with claim 1 in which said field-free space extends from said assembly to said gun mount.

3. A color-selection and post-deflection focus assembly in accordance with claim 1 in which said mesh electrode has a plurality of spaced rods for increasing the mechanical rigidity of said electrode.

4. A color-selection mask and post-deflection focus assembly in accordance with claim 1 in which said parallax mask is the intermediate one of said lens electrodes and in which the end ones of said lens electrodes are both formed of metallic mesh.

5. A color-selection mask and post-deflection focus assembly in accordance with claim 1 in which said mesh electrode is under tension which distorts the electrode from a desired configuration and in which the potential difference established on the lens electrodes creates an electrostatic force to overcome said tension and restore said electrode to said desired configuration.

6. A color-selection mask and post-deflection focus assembly in accordance with claim 1 in which said screen has a repeating pattern of phosphor strips each of which emits light of an individual color and in which said parallax mask is a grid of parallel wires disposed in the same direction as said phosphor strips.

7. A color-selection mask and post-deflection focus assembly in accordance with claim 6 in which said lens electrodes are essentially cylindrical sections.

8. A color-selection mask and post-deflection focus assembly in accordance with claim 6 in which said parallax mask is the intermediate lens electrode while the end electrodes are formed of metallic mesh and in which the terminal portions of said mask are electrostatically shielded by a conductive housing maintained at the same direct-current potential as said end electrodes.

9. A color-selection mask and post-deflection focus assembly for a color cathode-ray tube having a gun mount at one end and a line-type color phosphor screen at the opposite end comprising:

three lens electrodes aligned along but disposed transversely of the longitudinal axis of said tube in sub- 9 stantially spaced relation between said gun mount and said screen, the center one of said electrodes being a lenses to focus electrons exiting from said assembly to said particular portions of said screen.

grid of parallel wires disposed in the same direction as the phosphor strips of said screen and providing apertures for admitting electrons from said gun mount to particular portions of said screen dependent on the References Cited UNITED STATES PATENTS angle at which said electrons enter said assembly, the 2315367 3/1943 Epsteip 315 15 2,431,113 11/1947 Glyptis et a1 31382 X remalnmg two lens electrodes being formed of metal- 3 047 768 7/1962 McNaney 315 31 11c mesh having mterstrtial dlrnenslons small com- 5,076,121 1/1963 Stone 315 31 X pared with the aperture dimensions of said wire grid;

and means for establishing the end ones of said electrodes, at a common direct-current potential with said gun mount, and said screen to cause said assembly to be in a field-free space and for establishing the intermediate one of said electrodes at a direct-current 15 potential diflFerence relative to said end electrodes to 313-82 constitute therewith a system of converging electron RODNEY D. BENNETT, Primary Examiner.

MALCOLM F. HUBLER, Assistant Examiner.

US. Cl. X.R. 

